The present invention relates to an electroluminescent display and its drive method for displaying in color by using elements such as organic electroluminescent elements.
Heretofore, matrix displays are already known wherein light-emitting elements made from a material such as organic electroluminescence are used. Such a conventional matrix display has a matrix (lattice) of a plurality of anode lines and a plurality of cathode lines, and a plurality of light-emitting elements each of which is connected to each of the intersections of the matrix of the anode lines and cathode lines. For display in full color, R (red), G (green), and B (blue) light-emitting elements are arranged in order in such a manner that these three light-emitting elements are formed in one group so as to constitute one pixel. Each light-emitting element to be connected to each intersection can be represented by an electroluminescent element E with the diode properties and the parasitic capacitance C connected in parallel to the electroluminescent element E as shown in FIG. 1 in the attached drawings.
An example of this type of conventional full-color matrix displays will be described referring to FIG. 2 to FIG. 4 in the attached drawings. A1 to A768 are anode lines and B1 to B64 are cathode lines, so arranged as to intersect each other. Light-emitting elements R, G, and B, which emit red, green, and blue color respectively, are connected to each of the intersections of these anode and cathode lines. These light-emitting elements R, G, and B are arranged respectively in such a regular manner that light-emitting elements of the same color are connected to the same anode line. That is, the layout is constituted in such a manner that the anode line A1 has 64 light-emitting elements of R connected thereto, the anode line A2 has 64 light-emitting elements of G connected thereto, and the anode line A3 has 64 light-emitting elements of B connected thereto. On the other hand, cathode lines have light-emitting elements of R, G, and B connected thereto repeatedly and sequentially. Thus, three light-emitting elements R, G, and B, which are adjacent to each other, form a unit pixel E as a group. As shown in the drawings, 16384 pixels of E1xe2x80x21 to E256xe2x80x264 are to be arranged in a matrix.
A cathode line scanning circuit 1 comprises scanning switches 51 to 564 for scanning cathode lines B1 to B64 in sequence. Each scanning switch 51 to 564 is connected at one end thereof to a reverse bias voltage Vcc of a constant-voltage power supply, while the other end thereof is connected to the ground (0 V). This reverse bias voltage Vcc acts to prevent emission of light-emitting elements connected to a cathode line B1 to B64 not being scanned.
An anode drive circuit 2 comprises constant-current power supplies 21 to 2768 and drive switches 61 to 6768 for selecting anode lines to be connected to the constant-current power supply 21 to 2768 out of anode lines A1 to A768. Turning any drive switch ON will allow the constant-current power supply 21 to 2768 to be connected to the anode line corresponding to the drive switch.
A anode reset circuit 3 comprises shunt switches 71 to 7768 for connecting the anode lines A1 to A768 to the ground (0 V).
A light-emission control circuit 4 is provided for controlling the cathode line scanning circuit 1, anode drive circuit 2, and anode reset circuit 3 in response to light-emission data to be input.
Referring to FIGS. 2 to 4, the operation of the full-color matrix display will be described. The operation to be described below is an example wherein the cathode line B1 is scanned to cause a pixel E1xe2x80x21 to emit light and then the cathode line B2 is scanned to cause the pixel E2xe2x80x22 to emit light. In addition, for the sake of understanding the explanation, light-emitting elements which are emitting light are shown with diode symbols, while light-emitting elements which are not emitting light are shown with capacitor symbols.
FIG.2 shows the state wherein the pixel E1xe2x80x21 is emitting light. In this state, the cathode line B1 is being scanned with a scanning switch 51 switched to the ground potential. Scanning switches 52 to 564 have been switched to the constant-voltage power supply and thus the cathode lines B2 to B64 are subjected to the reverse bias voltage Vcc. On the other hand, the anode lines A1 to A3 are connected to the constant-current power supply 21 to 23 by means of the drive switches 61 to 63 and the shunt switches 71 to 73 are made open. Other anode lines A4 to A768 are connected to the ground potential by means of the shunt switches 74 to 7768 with the drive switches 64 to 6768 made open.
Thus, in the state shown in FIG. 2, only pixel E1xe2x80x21 is forward-biased in which a driving current is flowing in the direction shown by the arrow from the constant-current power supply 21, caused to emit light. The parasitic capacitance of the pixel E1xe2x80x21 is charged in the forward direction.
In this case, the light-emitting elements R, G, and B in pixels E1xc2x02 to E1xe2x80x264 are connected to the constant-power supplies 21 to 23. However, since the cathode lines are connected to the constant-voltage supplies so as to be kept at the reverse bias voltage Vcc, the voltage across the both sides of the light-emitting elements are almost 0V and thus these light-emitting elements do not emit light. In addition, the pixels E2xe2x80x21 to E256xe2x80x21 are connected at the both sides thereof to the ground potential and thus do not emit light. Furthermore, the pixels E2xe2x80x22 to E256xe2x80x264 are reverse-biased and thus do not emit light with the parasitic capacitance of the light-emitting elements charged in the reverse direction as shown in the drawing (by hatching the capacitors).
After the cathode line B1 has been scanned and before the cathode line B2 is started to be scanned, all the anode lines A1 to A768 and cathode lines B1 to B64 are once shunted to the ground potential to be reset to 0V. That is, as shown in FIG. 3, all the drive switches 61 to 6768 are turned OFF, while all the scanning switches 51 to 564 and all the shunt switches 71 to 7768 are switched to the ground potential. Since this causes all the anode and cathode lines to become the same potential of 0V, all the charges which were charged in each light-emitting element will be discharged.
Then, as shown in FIG. 4, the cathode line B2 is started to scan. That is, only the scanning switch 52 corresponding to the cathode line B2 is switched to the ground potential with other scanning switches 51, 53 to 564 connected to the reverse bias voltage Vcc and drive switches 64 to 66 switched to the constant-current power supply 24 to 26. Consequently, the anode lines A4 to A6 are driven, shunt switches 71 to 73, 77 to 7768 are turned ON, and the anode lines A1 to A3, A7 to A768 are turned in the potential thereof to 0V.
As described above, since all the light-emitting elements have zero electric charge in the moment of switching each switch, the anode lines A4 to A6 has the potential of Vcc (more accurately 63/64 Vcc). This allows the light-emitting elements of the pixel E2xe2x80x22, which is to emit light subsequently, to be charged at a dash by charging currents from a plurality of paths shown by the arrows in FIG. 4, the parasitic capacitance of each light-emitting element is charged instantaneously, and thus these light-emitting elements emit light at predetermined instantaneous luminance.
Concerning the reset operation mentioned above, the present applicant has already proposed the method disclosed in Japanese Patent Application Laid Open No. 9-232074. The reset driving method disclosed in the above patent publication solves the problem that the reverse-direction electric charges of pixels E2xe2x80x22 to E2xe2x80x264 charged at the time of scanning the cathode line B1 cause light-emitting elements on the cathode line B2 to be delayed in rising for emitting light when the cathode line B2 is scanned.
That is, in order to allow light-emitting elements to emit light at predetermined instantaneous luminance, the voltage across the both sides of the respective light-emitting element needs to be built up to a certain specified value. For this purpose, the parasitic capacitance of the light-emitting element must be charged by a predetermined amount of charge. Canceling the reverse-direction charges charged to pixels E2xe2x80x22 to E2xe2x80x264 will enable the anode lines A4 to A6, which are driven when the cathode line B2 is scanned, to turn the voltages thereof to Vcc instantaneously (that is, the voltage across the both ends of each light-emitting element of the pixel E2xe2x80x22 can be turned approximately to Vcc in an instant), and whereby a quick charge is made possible to the light-emitting elements of the pixel E2xe2x80x22.
According to the conventional reset driving method, each light-emitting element of the pixel E2xe2x80x22 which is to emit light in the moment the cathode line B2 is about to be scanned has approximately a voltage of Vcc across the both sides of the light-emitting element.
However, each light-emitting element of R, G, and B has a difference in the luminescent material and in the element structure, and thus has different luminance-voltage characteristics in most cases. According to the conventional reset driving method, a light-emitting element of R, G, and B, which has the specified value of a voltage across the both sides thereof closer to Vcc, is allowed to emit light more quickly at predetermined instantaneous luminance. However, there may be a case in which the specified value of a voltage across the both sides of a light-emitting element is considerably greater than Vcc. In this case, the method has such a problem that the light-emitting element needs to be charged more by the current flowing from the constant-current power supply for light emission and is consequently delayed in rising for light emission. The method has also such a problem that a driving method such as the pulse-width modulation drive, in which gradations are expressed by the duration of light emission within a scan period, provides bad linearity of gradations.
The object of the present invention is to solve the forgoing problems involved in conventional methods and to provide an electroluminescent display which allows each of R, G, and B light-emitting elements to be built up simultaneously for emitting light at predetermined instantaneous luminance in order to improve the reproducibility of gradations when addressed by the pulse-width modulation drive.
Another object of the present invention is to provide a method of driving the electroluminescent display.
To achieve the first object, according to first aspect of the present invention there is provided an electroluminescent display in which a matrix of anode lines and cathode lines is provided, either one of which is used as scanning lines and the other as drive lines, a light-emitting element is connected to an intersection of the scanning line and the drive line in such a manner that light-emitting elements having the same color of red, green and blue are connected to each drive line, and while scanning a scanning line, a power supply is connected to a predetermined drive line in response to the scan on the scanning line, hereby causing a light emission by the light-emitting element connected to the intersection of the scanning line and the drive line, wherein a charging means is provided for charging at least any one of said red, green and blue light-emitting elements in the duration between the end of a scan and the start of the subsequent scan, and the charging means charges different amounts of charge to the red, green and blue light-emitting elements.
The charging means may be intended for charging all of said light-emitting elements.
Each of the red, green and blue light-emitting elements may have a different specified voltage across the both ends thereof under steady light-emission conditions.
The charging means may be arranged in such a manner that positive electric charge is charged to an element of said red, green and blue light-emitting elements which has the highest value of said specified light-emission voltage, no charge is charged to an element having the second highest voltage, and negative electric charge is charged to an element having the lowest value of said specified light-emission voltage.
According to second aspect of the present invention there is provided an electroluminescent display in which a matrix of anode lines and cathode lines is arranged, either one of which is used as scanning lines and the other as drive lines, a light-emitting element is connected to an intersection of the respective scanning line and the respective drive line in such a manner that light-emitting elements having the same color of red, green and blue are connected to each drive line, and while scanning a scanning line, a power supply is connected to a predetermined drive line in response to the scan on the scanning line, hereby causing a light emission by the light-emitting element connected to the intersection of the scanning line and the drive line, wherein the scanning lines are made connectable to either one of a first constant-voltage power supply or a ground means, the drive lines are made connectable to either one of the power supply, ground means and a second constant-voltage power supply for charging electric charge to the light-emitting elements, the scanning lines are connected to the ground means, the drive lines are connected to the second constant-voltage power supply during the duration between the end of a scan and the start of the subsequent scan, and the second constant-voltage power supply applies a different voltage depending on which element of the red, green and blue light-emitting elements is to be connected thereto.
Each of the red, green and blue light-emitting element may have a different specified voltage across the both ends thereof under steady light-emission conditions.
The second voltage supply is provided only for drive lines to which light-emitting elements having the highest value and the lowest value of the light-emission specified voltage are connected among the red, green and blue light-emitting elements, and the light-emitting element having the highest value of the specified voltage may be forward-biased and the light-emitting element having the lowest value of the specified voltage may be reverse-biased.
In the scan period during which an arbitrary scanning line is being scanned, the ground means may be connected to the scanning line which is being scanned, whereas the first constant-voltage power supply may be connected to the scanning line which is not being scanned; and the power supply may be connected to a drive line to which a light-emitting element to be emitting light is connected, whereas the ground means may be connected to a drive line to which a light-emitting element not to be emitting light is connected.
The light-emitting elements may be formed of organic electroluminescent materials.
According to third aspect of the present invention the second object of the present invention is attained by providing a method of driving an electroluminescent display in which a matrix of anode lines and cathode lines is formed, either one of which is used as scanning lines and the other as drive lines, a light-emitting element is connected to an intersection of the respective scanning line and the respective drive line in such a manner that light-emitting elements having the same color of red, green and blue are connected to each drive line, and while scanning a scanning line, a power supply is connected to a predetermined drive line in response to the scan on the scanning line, hereby causing a light emission by the light-emitting element connected to the intersection of the scanning line and the drive line, wherein different amounts of charge is charged to the red, green and blue light-emitting elements during the duration between the end of a scan and the start of the subsequent scan.
With the method of driving an electroluminescent display according to the present invention, during the duration between the end of scanning a scanning line and the start of scanning the subsequent scanning line, positive electric charge may be charged to the light-emitting element, among the red, green and blue light-emitting elements, having the highest value of the light-emission specified voltage, a voltage across the both ends thereof at the state of steady light emission. The light-emitting element having the second highest value of specified voltage may have no electric charge charged, whereas the one having the lowest value of the light-emission specified voltage may have negative electric charge charged.
According to a fourth aspect of the present invention there is provided a method of driving an electroluminescent display in which a matrix of anode lines and cathode lines is formed, either one of which is used as scanning lines and the other as drive lines, a light-emitting element is connected to an intersection of the respective scanning line and the respective drive line in such a manner that light-emitting elements having the same color of red, green and blue are connected to each drive line, and while scanning a scanning line, a power supply is connected to a predetermined drive line in response to the scan on the scanning line, hereby causing a light emission by the light-emitting element connected to the intersection of the respective scanning line and the respective drive line, wherein the scanning lines are made connectable to either one of a first constant-voltage power supply or a ground means, the drive lines are made connectable to either one of the power supply, ground means and a second constant-voltage power supply for charging electric charge to the light-emitting elements, in the scan period during which an arbitrary scanning line is being scanned, the ground means is connected to the scanning line which is being scanned, whereas the first constant-voltage power supply is connected to the scanning line which is not being scanned, the power supply is connected to a drive line to which a light-emitting element to be emitting light is connected, whereas the ground means is connected to a drive line to which a light-emitting element not to be emitting light is connected, the scanning lines are connected to said ground means, the drive lines are connected to the second constant-voltage power supply during the duration between the end of a scan and the start of the subsequent scan, and the second constant-voltage power supply applies a different voltage depending on which element of the red, green and blue light-emitting elements is to be connected thereto.
The second voltage supply is provided only for a drive line to which the light-emitting element having the highest value and the lowest value of the light-emission specified voltage is connected among the red, green and blue light-emitting elements. The light-emitting element having the highest value of the specified voltage may be forward-biased and the light-emitting element having the lowest value of the specified voltage may be reverse-biased.
The light-emitting elements may be formed of organic electroluminescent materials.
In the present invention, in the moment a scan is switched from an arbitrary cathode line to the subsequent line, each of the R, G and B light-emitting elements is electrically charged depending on the element. Therefore, each of R, G, and B light-emitting elements can be built up simultaneously for emitting light at predetermined instantaneous luminance so as to improve the reproducibility of gradations when driven by the pulse-width modulation drive.